Step 2: Cut teeth off of chain ring

Step 3: Replace old chain ring with toothless one

Next step is to remove the existing large chain ring from the crank set. Using a 5mm hex key, carefully unbolt the chain ring. Be careful as you remov...

I built an inexpensive chain guard for my bike. Getting your pant leg or shoelaces caught in the chain while riding is at the very least annoying, and can potentially be very dangerous. The usual pant clip does keep your pant cuff out of the chain, but does not keep shoelaces safe.

I ride a fairly heavy bike, and realized I never used the largest gear on the front derailleur. I decided to replace it with a smooth plate, which acts a chain guard. I made this at TechShop.

Step 1: Mount the chain ring on the lathe

The existing big chain ring was a 44 tooth one, but I never rode fast enough to shift up into that range. I found a 48 tooth gear in my parts bin, and decided to repurpose it.

Since I had access to a metal lathe at TechShop, I decided that was the tool I wanted to use. I used the four jaw chuck on the metal lathe, since this crank set uses four spokes to locate the gears. It is important to center the part when using a four jaw chuck, hence the dial gauge on the left in the photo. Centering is done via small adjustments to each of the jaws, measuring the runout between adjustments. This was by far the longest step in the process.

Make sure the chain ring is held in tightly, but not too tight or the gear will flex out of round.

Step 2: Cut teeth off of chain ring

Move the cutter in close and turn on the lathe. Slowly slice the teeth from the outside of the chain ring. It will be pretty noisy initialy, as it is an interrupted cut. When the sound becomes uniform, you know you have cut all the teeth off. Stop the lathe, inspect your work, and unmount it when you are satisfied with the cut. I cleaned up the edge with a small file.

This is very nice, to be sure. An elegant solution. I happen to read this as my chain guard just fell off. I always use the highest gear, but there are bolts for a crap plastic chain guard. I will see about making a new one and whether there is an instructable there. Thanks for sharing. (ps - is the Tech Shop a Hackerspace of sorts?)

Yes, the techshop is a hackerspace of sorts. Aren't those awesome? I remember having the idea when I was a kid that people should share tools like they share books. Comforting to know I'm not the only one who feels that way. At the time it never occurred to me that the knowledge shared would be even more valuable than the tools.

As a machinist your setup made me cringe. One hard spot or hard aluminum drag and you would have sent a literal saw blade off that lathe at high speed. A better setup would have been either a faceplate so you could take advantage of those four bolt holes (and other parts) or a smaller lathe (or 4 jaw chuck) with the jaws reversed and the body of the gear laid flat against the face of the chuck. Always safety first!

Be very careful with these, these are not meant to act as true chainring guards. They will not protect shins should you slip from pedals. A thin piece of metal (~3/32" thick) like this trimmed chainring with enough force behind it can do alot of damage. I've had one nasty experience with a similar setup previously used. True chainring guards are just barely larger OD than the OD of the chain on the chainring it's intended to protect.

That's an excellent point. More importantly, a real "bash guard," which is what this instructable produces, is made of much flimsier metal, so it would probably bend rather than cutting you as deep as this one. More importantly, I think they usually have a rolled-over edge to prevent cutting you as well.